Solid oxide fuel cells (SOFC) are a class of fuel cell characterized by the use of a solid oxide material as an electrolyte, which conducts oxygen ions from a cathode to an anode. At the anode, the oxygen ions combine electrochemically with hydrogen and/or carbon monoxide to form water and/or carbon dioxide, respectively. Unlike many other types of fuel cells, solid oxide fuel cells offer great prospects for the most efficient and cost-effective utilization of readily available fuels such as hydrocarbons and gasified coal or a wide variety of ample carbonaceous solids (e.g. municipal solid waste and biomass). Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs ranging from 100 W to 2 MW, at an energy efficiency of about 60 percent. Solid oxide fuel cells can have multiple geometries. A typical planar fuel cell design has a sandwich-type geometry, where a relatively dense electrolyte is sandwiched between a cathode and an anode. This sandwich type geometry facilitates the stacking of hundreds of cells in series, with each cell typically having a thickness on the order of a few millimeters. Because the ceramics used in conventional fuel cells have limited conductivities at low temperatures, stacks of cells typically run at temperatures ranging from about 500° C. to about 1000° C.
As illustrated by FIG. 1, operation of a fuel cell begins by a reduction of oxygen into oxygen ions at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode, where they can electrochemically combine with a fuel, such as a light hydrocarbon fuel (e.g., methane, propane and butane) to form water and carbon dioxide, releasing electrons at the anode, which flow through an external circuit, performing electrical work, back to the cathode.
The anode typically must be highly porous to enable fuel to flow towards the electrolyte. Like the cathode, the anode must conduct electrons with low resistivity and should have high ionic conductivity. A common anode material is a cermet made up of nickel mixed with the same ceramic material used for the electrolyte of the cell, which is typically yttria-stabilized zirconia (YSZ). The anode may be the thickest and strongest layer in each individual cell, because it has the smallest polarization losses and is often the layer used to provide mechanical support to the cell. The function of the anode is to oxidize the fuel efficiently. The electrochemical oxidation of hydrogen within the cell produces heat as well as water and electricity. If the fuel is a light hydrocarbon, such as methane, then another function of the anode is to act as a catalyst for steam reformation of the fuel into hydrogen and carbon monoxide. This additional function provides a benefit to a fuel cell stack because the reforming reaction is endothermic and provides internal cooling to the stack of individual cells.
The electrolyte of a fuel cell is typically a dense layer of ceramic oxide that conducts oxygen ions with preferably high ionic conductivity. However, to prevent leakage currents flowing between the anode and cathode, the electronic conductivity of the electrolyte should be as low as possible. The relatively high operating temperature of solid oxide fuel cells supports high oxygen ion transport through the electrolyte. Popular electrolyte materials include YSZ, scandia-stabilized zirconia (ScSZ)), strontium-magnesium doped lanthanum gallate (LSGM), samarium-doped ceria (SDC), and gadolinium-doped ceria (GDC). Detrimental reactions between YSZ electrolytes and cathode materials, such as lanthanum strontium cobalt ferrite (LSCF), can be prevented using a diffusion barrier/buffer layer (˜2 to 5 μm thick), such as samarium or gadolinium-doped ceria.
The cathode of a fuel cell is typically a thin porous layer on the electrolyte where oxygen reduction takes place. Cathode materials must be, at a minimum, electronically conductive. Currently, lanthanum strontium manganite (LSM) is the cathode material of choice for many commercial applications because of its compatibility with doped zirconia electrolytes. This compatibility includes a similar coefficient of thermal expansion (CTE) and a low chemical reactivity with YSZ. Unfortunately, LSM is a relatively poor ionic conductor, which means that an electrochemically active reaction for oxygen reduction is limited to a triple-phase boundary (TPB) where the electrolyte, air and cathode meet.
In order to increase the reaction zone for oxygen reduction beyond the TPB, a cathode material containing a composite of LSM and YSZ has been used because it has a relatively high electron and oxygen ion conductivity. One state-of-the-art cathode is a porous composite of (La0.8Sr0.2)0.95MnO3+δ (LSM) and the solid electrolyte 8% Y2O3-doped ZrO2 (YSZ), with the volumetric composition of the composite being approximately 40% pore, 35% LSM and 25% YSZ. This composite has been shown to be advantageous relative to porous LSM, because YSZ is a better ionic conductor than LSM. The active area of porous LSM cathode is limited to regions close to the LSM cathode-YSZ electrolyte interface, but in the LSM/YSZ composite, the active area is extended beyond the cathode-electrolyte interface by the availability of YSZ in the porous cathode, which acts as a fast ionic transport pathway.
Unfortunately, the performance of the composite. LSM/YSZ cathode may be limited by the fact that there may be only about 30% connectivity for the YSZ within the composite, which means that inactive YSZ clusters will exist at points relatively far from the electrolyte. This connectivity problem may be overcome by creating a cathode having a non-random composite microstructure using, for example, an infiltration technique. One such infiltration technique includes firing a porous, single phase YSZ backbone, and then infiltrating liquid precursors of LSM into the porous YSZ backbone before re-firing. This technique has been shown to yield a thin dense coating (about 60 nm) of LSM over the YSZ. Unfortunately, even this very thin coating of LSM is typically much less electrochemically active than a morphology with an exposed triple-phase boundary. Moreover, the LSM coating may suffer relatively high sheet resistance at moderate cell current densities.
As an alternative to infiltrating a YSZ backbone with LSM precursors, porous LaxSr1-xCoyFe1-yO3+δ (LSCF) has been used as a cathode in solid oxide fuel cell applications because it has much higher ionic and electrical conductivity relative to LSM at intermediate temperatures. But, because LSCF reacts adversely with YSZ, a buffer layer of ionically conductive Gd- or Sm doped ceria (CeO2) can be provided between LSCF and YSZ. Nonetheless, the long-term stability and the catalytic activity of LSCF for oxygen reduction are typically less than that of LSM.